Alkali metal aluminosilicates, both crystalline (zeolites) and amorphous, are effective detergency builders which can be used to replace sodium tripolyphosphate (STP) in detergent powders, but they do not possess an ability comparable to that of STP to contribute to the structure of a spray-dried powder. Alkali metal silicates are frequently included in detergent powders as structurants providing a robust surface to the spray-dried particle, to reduce washing machine corrosion and to increase alkalinity. It is well known, however, that if aluminosilicate and silicate are together in a detergent slurry they can interact unfavourably: agglomeration of the aluminosilicate occurs to give powders containing large particles which are slow to disperse in the wash liquor, giving reduced washing performance.
For this reason attempts have been made to reduce the level of sodium silicate included in spray-dried powders built with aluminosilicates, but this tends to cause deterioration of the flow properties (dynamic flow rate, compressibility) of the powders. Alternative structurants are therefore needed to compensate for the reduced silicate level.
EP 61 295B (Unilever) describes and claims a spray-drying process for the preparation of crisp, free-flowing detergent powders containing less than 6% by weight of phosphate (calculated as phosphorus) and less than 4% by weight of sodium silicate. According to that process, the slurry which is spray-dried to form a powder includes a water-soluble salt of succinic acid, preferably sodium succinate. The succinic acid salt may be wholly or partially neutralised.
We have now found that even better powder properties may be achieved using succinic acid salts incorporated at levels of 0.5 to 2.5% by weight as structurants, if there is also included in the slurry a film-forming polymeric polycarboxylate, in an amount of from 0.5 to 10% by weight based on the final powder.
Ep 1310A (Procter & Gamble) discloses spray-dried zero-phosphate or low-phosphate detergent powders built with zeolite and containing materials such as sodium citrate as supplmentary builders or pH regulators. The Examples contain sodium citrate levels ranging from 6 to 20% by weight, and also contain varying amounts of methyl vinyl ether/maleic anhydride copolymerrs (Gantrez (Trade Mark) AN 119 and AN 136 ex GAF); it is stated that the sodium citrate may be replaced by sodium succinate. EP 1853B (Procter & Gamble) contains a similar disclosure, with Examples containing 4 to 15% by weight of sodium citrate and 0.8 to 2% by weight of Gantrez polymer. In those Examples the sodium citrate, or the sodium succinate which may replace it, is functioning as a detergency builder or pH regulator.
Our discovery, on the other hand, is concerned with the structurant properties of succinates, at levels too low for building efficacy, when combined with polymers. Citrates do not have these properties.